The weather resistance of polyamides, that is, their resistance to the simultaneous or alternating action of UV radiation and elevated temperatures in the presence of oxygen and moisture, is insufficient for a number of applications. With the exception of the so-called completely aromatic polyamides (e.g. Kevlar.RTM.), this deficiency concerns practically all known polyamides, the homopolyamides PA-6, PA-66, PA-12, PA-1i, PA-69, PA-6.10 etc. just as well as the copolyamides accessible from any aliphatic monomer combinations. However, this also affects the partially crystalline or amorphous, partially aromatic polyamides derived from aromatic dicarboxylic acids, aliphatic and/or cycloaliphatic diamines or the corresponding salts and, if applicable, from any other polyamide-forming monomers.
It has long been common practice in the polymerization or polycondensation of polyamides to use amines, mono- or dicarboxylic acids as chain terminators; usually, carboxylic acids such as e.g. acetic acid, benzoic acid or adipic acid are used for this purpose. However, other aliphatic or cycloaliphatic diamines are also used if it is a matter, for example, of improving the adhesion of a polyamide to carboxyl- or carboxylic-acid anhydride-functional polymers. The amine termination can be used with success on polymer mixtures, e.g. blends of PA-6 or PA-12 with anhydride-functional poly(2,6-dimethylphenylene oxide) or on polymer compounds composites!, e.g., multi-layer pipes or multi-layer foils. Further effects can be achieved by the type and amount of the chain terminator which go beyond a pure control of the reaction rate during the polymerization or polycondensation and of the molecular weight or degree of polymerization of the particular polyamide to be achieved thereby. Thus, calculations have shown in conjunction with the hydrolytic polymerization of laurinlactam that the molecular weight distribution MW/MN of the PA-12 depends on the functionality and the amount of the chain terminator and on the conversion of the polymerization (see "Angewandte Makromolekulare Chemie 34" (1973), pp. 135-152 and 153-163). The authors demonstrate, among other things, that closer distributions molecular weight distributions result from the use of a difunctional chain transfer agent such as e.g. adipic acid in all chain terminator concentrations and conversions relevant for the practice than in the case of a monofunctional termination or in the case of uncontrolled PA-12. These findings should also be at least qualitatively transferable to other polyamides, e.g. PA-6. The results of the cited works can have considerable practical significance since it is known that the processability of a polyamide is influenced by its molecular weight distribution. Experience teaches that especially in the spinning of polyamides, primarily PA 6 and PA-66, comparatively close molecular weight distributions are advantageous as concerns the capability of being spun (spinning speed and spinning safety) and the fiber properties (strength and uniformity).
In addition to the "typical" chain transfer agents cited above (benzoic acid, acetic acid, adipic acid) there are other substances which act as chain terminators and are suitable on account of their special molecular structure for improving the thermal stability and, if applicable, photostability of polyamides. These substances include, for the one, carboxyl- and ester- or amine-functionalized, sterically hindered phenols in which the phenolic OH group is sterically shielded by one or two voluminous alkyl groups, e.g. 3,5-di-t-butyl-4-hydroxybenzoic acid, 3-(3,5-di-t-butyl-4-hydroxyphenyl)-propionic acid or 3-(3,5-di-t-butyl-4-hydroxyphenyl)-1-aminopropane. For the other, they include 4-amino-2,2,6,6-tetraalkylpiperidines which can be further substituted on the N atom of the piperidine ring, e.g. 4-amino-2,2,6,6-tetramethylpiperidine, 1called triacetone diamine for short, or 4-amino-1-alkyl-2,2,6,6-tetramethylpiperidine with C.sub.1 -C.sub.18 alkyl groups or with a benzyl group in position 1.